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Don Draper's current gal on the side tells THR about show secrecy keeping her from the premiere party, her mother's reaction to her racier scenes and the recent mini-reunion for her iconic comedy.

Mad Men's attention may currently be fixed on the rapidly changing situation at Sterling Cooper Draper Pryce, but there's still the matter of the unhappy doctor's wife keeping a key under the mat several floors below the Drapers' Upper East Side apartment.

Linda Cardellini raised more than a few eyebrows when her quiet new character was revealed to be sleeping with relapsing lothario Don Draper (Jon Hamm) during the season six premiere -- largely because, in typical Mad Men fashion, no one even expected to see her on the show. Sylvia Rosen has since been revealed to be one of Draper's more complicated mistresses.

Not only has her geographic proximity led to awkward laundry room run-ins with Mrs. Draper (Jessica Pare), Sylvia's devout Catholicism makes her an unlikely instigator in the fling Don says he wants to quit.

Cardellini, who was recently featured in The Hollywood Reporter's comedy issue with her Freaks and Geeks boss Paul Feig, spoke with THR about her first big gig since having her daughter in 2012, navigating Mad Men's obligatory gag order and the lingering appeal of her brilliant-but-canceled comedy.

The Hollywood Reporter: Every job on Mad Men requires a certain vow of silence. How discreet did you have to be when you started working on the show?

Linda Cardellini: I couldn’t tell anyone. I didn’t tell anyone -- which made it really different for me in terms of people around me knowing what I am doing and who I am working with. It was fun, though. As a fan, I enjoy not knowing. Being on the show, there are things that are no longer surprises for me, and I miss that element. I am more then happy to keep the secret. It adds to the show. It's fun to watch something and be involved and be surprised by things.

THR: It's funny how the show can still pull off casting surprises. What did you think of the reaction after the premiere?

Cardellini: I was happy people were surprised, and happy people noticed. I was also excited because in the first script I read I had no lines and was in the background. By the second script [of the two-part episode], it was clear what my character's purpose was.

THR: Who told you, 'We're so excited to have you on the show, but you can't come to the party?'

Cardellini: Pretty much everybody [laughs]. I was fine with it. By that point, I had kept the secret so long I didn’t want anyone to spoil it.

THR: What's the reaction from your friends and family?

Cardellini: They really like seeing me in period costume, because that's very new for me. My mom asked me if I was ever going to get out of that bed.

THR: Your mother didn't even know.

Cardellini: No, my parents knew I was working on something, but they just waited for me to tell them. They are really excited now.

THR: In hindsight, Sylvia's crucifix is really telling, but it's not something that immediately jumps out at you.

Cardellini: Which is also a great thing. It's not in your face, but she is always wearing it.

THR: Sylvia appears to be really stoic about the moral implications of the affair, but she's a devout Catholic. Were you surprised by her reaction when Megan alluded to wanting an abortion?

Cardellini: I knew it was part of her character, that she was religious, which I think is interesting for her because she is doing something that is clearly sinful in her mind. Although she talks about feeling guilty about it, her actions speak otherwise. She's a cheater like Don, but she handles it differently – even though she is doing the same thing as Don. She says she feels guilty, but is she really guilty or is she not?

THR: This is the first time that Don seems to be the one least comfortable with cheating. What do you think is different about this affair?

Cardellini: That is so hard to answer because I know more then I can say. I think that ... no, I don’t know how to answer that one.

THR: You and Busy Phillips recently joined Freaks and Geeks creator Paul Feig for THR's Comedy Issue photo shoot. Do you want to do more comedy?

Cardellini: Yeah, I would love to do some comedy again. It's been a while; it would be fun to do something comedic, but at the same time it depends on the role. I loved Bridesmaids. Kristen Wiig is amazing and hilarious, and I thought Paul did an amazing job.

THR: So many actors in Paul and Judd Apatow's work come from Freaks and Geeks. What do you think of the show's longevity?

Cardellini: I love the fact that it didn’t die when it was canceled. Back at that point there weren’t even that many shows that went to DVD. There was a chance that that show could have disappeared and never been seen again, so the idea now that it lives on DVD and on Netflix and that people can pick up and watch it as though it's still on the air is fantastic to me. We were so crushed when it was canceled, and now that people have found it again, and continue to find it, it just creates this fan base that is so amazing and rewarding.

THR: You were so young when you played Lindsay Weir. Did you have any idea your short time playing her would be such a definitive part of your career?

Cardellini: I think with everything you work on, you feel it's special. We all had such an incredible time, and it was really a special moment in all of our lives -- especially at the beginning of our careers. We were really the underdog of the network. We were the freaks and geeks of TV, and I think we were very proud of what we were doing. I don’t think you can ever plan for people loving something like that.